The present invention relates to a machine for holding a workpiece and a holding unit usable in the machine.
The machines of the above kind are widely used to hold and position a vehicle body component part, such as a vehicle body panel, when welding to the other vehicle body component parts.
FIG. 1 shows a known jig machine used in welding operation in a vehicle body assembly line which is designed to hold and position a front pillar W as a workpiece. As shown, there are four posts 2 standing upright on a base 1 and secured thereto by bolts 10. Supported on a top portion of each of the four posts 2 is a workpiece supporting device 3, viz., a gage plate, with a cutout 3a having a shape suitable for reception of the workpiece. In order to assist the machine in positioning the workpiece, a locate pin 5 is attached to one of the posts 2 via a mounting bracket 4. In order to fixedly keep the workpiece within the cutouts 3a of the workpiece supporting devices 3, each of the posts 2 has a pivotable clamp member 7 which is actuated by an actuator 8 to move to a closed position as illustrated where it presses the workpiece W against the cutout 3a defining surface, so as to clamp the workpiece W therebetween. After the workpiece W is positioned as illustrated, a spot welding operation is performed by a welding robot, not illustrated. Designated by the reference numeral 9 are locate pins which are used to position the posts 2 relative to the base 1 before they ar secured to the base 1 by means of the bolts 10.
The accuracy with which the workpiece is welded in assembling vehicle bodies is largely dependent on the accuracy with which the machine is adjusted. The adjustment is made by fixing the workpiece supporting device 3 to each of the posts 2 and then positioning the posts 2 relative to the base 1 before fixedly fastening the mounting bolts 10. Since the shape of each cutout 3a and that of each clamp member 7 cannot be used for holding and positioning another different workpiece, its use is confined to the single sort of workpiece. The known machine, therefore, cannot be used for different workpieces without replacing it as a whole including the base and thus it poses a problem that it cannot be used in a flexible manufacturing system where different types of automobiles are assembled in the same assembly line continuously.
In order to alleviate the problem encountered in the prior art, it has been proposed to arrange side-by-side a plurality of posts 15 with workpiece supporting devices 17 formed with different cutouts which are pivotable and selectively put into an upright operative position where it can support and position the corresponding workpiece, as shown in FIG. 2. This proposal brings about a bulky and space occupying construction as a whole and thus is not considered to provide a satisfactory solution to the problem discussed above.